Trichoplax Adhaerens - Locomotion

Locomotion

Placozoa can move in two different ways on solid surfaces: First, their ciliated crawling sole lets them glide slowly across the substrate; second, they can change location by modifying their body shape, as an amoeba does. These movements are not centrally coordinated, since no muscle or nerve tissues exist. It can happen that an individual moves simultaneously in two different directions and consequently divides into two parts.

It has been possible to demonstrate a close connection between body shape and the speed of locomotion, which is also a function of available food:

• At low nutrient density, the spread-out area fluctuates slightly but irregularly; speed remains relatively constant at about 15 micrometres per second.

• If nutrient density is high, however, the area covered oscillates with a stable period of about 8 minutes, in which the greatest extent reached by the organism can be as much as twice the smallest. Its speed, which remains consistently below 5 micrometres per second, varies with the same period. In this case, a high speed always corresponds to a reduced area, and vice versa.

Since the transition is not smooth but happens abruptly, the two modes of extension can be very clearly separated from one another. As a simplification, Trichoplax adhaerens can be modeled as a nonlinear dynamic system that is far from thermodynamic equilibrium.

The following is a qualitative explanation of the animal's behavior:

• At low nutrient density, Trichoplax maintains a constant speed in order to uncover food sources without wasting time.

• Once such a source is identified by high nutrient density, the organism increases its area in regular increments and thereby enlarges the surface in contact with substrate. This enlarges the surface through which nutrients can be ingested. The animal reduces its speed at the same time in order to actually consume all of the available food.

• Once this is nearly completed, Trichoplax reduces its area again to move on. Because food sources such as algal mats are often relatively extensive, it is reasonable for such an animal to stop moving after a brief period in order to flatten out again and absorb nutrients. Thus Trichoplax progresses relatively slowly in this phase.

The actual direction in which Trichoplax moves each time is random: if we measure how fast an individual animal moves away from an arbitrary starting point, we find a linear relationship between elapsed time and mean square distance between starting point and present location. Such a relationship is also characteristic of random Brownian motion of molecules, which thus can serve as a model for locomotion in the Placozoa.

Small animals are also capable of swimming actively with the aid of their cilia. As soon as they come into contact with a possible substrate, a dorsoventral response occurs: the dorsal cilia continue to beat, whereas the cilia of ventral cells stop their rhythmic beating. At the same time, the ventral surface tries to make contact with the substrate; small protrusions and invaginations, the microvilli found on the surface of the columnar cells, help in attaching to the substrate via their adhesive action.